On Mon, Jul 11 2022, "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx> wrote: > * Peter Maydell (peter.maydell@xxxxxxxxxx) wrote: >> On Mon, 11 Jul 2022 at 14:24, Dr. David Alan Gilbert >> <dgilbert@xxxxxxxxxx> wrote: >> > But, ignoring postcopy for a minute, with KVM how do different types of >> > backing memory work - e.g. if I back a region of guest memory with >> > /dev/shm/something or a hugepage equivalent, where does the MTE memory >> > come from, and how do you set it? >> >> Generally in an MTE system anything that's "plain old RAM" is expected >> to support tags. (The architecture manual calls this "conventional >> memory". This isn't quite the same as "anything that looks RAM-like", >> e.g. the graphics card framebuffer doesn't have to support tags!) > > I guess things like non-volatile disks mapped as DAX are fun edge cases. > >> One plausible implementation is that the firmware and memory controller >> are in cahoots and arrange that the appropriate fraction of the DRAM is >> reserved for holding tags (and inaccessible as normal RAM even by the OS); >> but where the tags are stored is entirely impdef and an implementation >> could choose to put the tags in their own entirely separate storage if >> it liked. The only way to access the tag storage is via the instructions >> for getting and setting tags. > > Hmm OK; In postcopy, at the moment, the call qemu uses is a call that > atomically places a page of data in memory and then tells the vCPUs to > continue. I guess a variant that took an extra blob of MTE data would > do. Yes, the current idea is to extend UFFDIO_COPY with a flag so that we get the tag data along with the page. > Note that other VMMs built on kvm work in different ways; the other > common way is to write into the backing file (i.e. the /dev/shm > whatever atomically somehow) and then do the userfault call to tell the > vcpus to continue. It looks like this is the way things will work in > the split hugepage mechanism Google are currently adding. Hmm... I had the impression that other VMMs had not cared about this particular use case yet; if they need a slightly different mechanism, it would complicate things a bit.
